Not knowing how you filed in previous years as to the room rental, the depreciation not taken does not matter. If you were entitled to take depreciation, and from what you are describing you were, the IRS goes by the phrase "allowed or allowable"; meaning even if you did not take it you were entitled to and must account for it as such.
What you need to do is go back and amend the previous 2 years returns to account for the depreciation not taken, then carry the adjusted basis forward to this year for the calculation of gain, or partial gain to be claimed in this case as only a partial part of the house was rental property subject to depreciation and carryback. The remainder of the house that was your personal residence is allowed the exclusion on the sale of a personal residence.
@hshah15006
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